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CERN | Geneva | Switzerland

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Dark matter: No model, just guesses

On the last day of the International Conference on High Energy Physics dark matter took a central seat.

As many of you know, ourselves, the earth, all stars and galaxies are made of atoms. These atoms emit light when they are excited and that is how astronomers can explore the vast universe. But this matter only accounts for 4% of the content of the universe while dark matter makes up 24% of it. An unknown type of energy dubbed “dark energy” makes up the remaining 76%.

Dark matter was discovered in 1933 by Swiss physicist Fritz Zwicky. But to this day, scientists still don’t know what it is made of. This matter emits no light, which is why it was called “dark matter”.

Dark matter seems to react only to gravitational force and this is how it was discovered. Zwicky realized there was more matter in the universe than what was visible from the light emitted by stars and galaxies. This matter creates a much stronger gravitational field than what can be accounted for if you only rely on visible matter.

Neal Weiner, a theorist from New York University, started his lecture saying that contrary to the Higgs boson, for dark matter “we have no model, only guesses”. There is nothing within the Standard Model of particle physics to account for dark matter. This is one key reason we physicists are all convinced there is a bigger theory hiding behind the current known one.

So theorists and experimentalists are in the dark… As Neal stressed, there are many manifestations of dark matter. Different experiments observe strange signals where dark matter could be the explanation. But formulating an explanation is far from being trivial.

For example, several experiments have reported seeing more positrons than electrons coming from outer space. Positrons are the antimatter for electrons. Recently, the Pamela and the Fermi experiments both saw an excess of positrons, particularly at high energy. Given that the universe is made of matter, one needs to explain where these anti-electrons come from.

Some astronomers think it could be produced by pulsars but the jury is still out on this. Others argue that dark matter could annihilate into a pair of electron and positron, creating more positrons than expected. But it is not easy to cook up a theory that would do that. Hopefully, new data will come in 2013 from the Planck satellite to resolve this issue.

The DAMA/Libra experiment has been reporting a loud and clear signal (8.7 sigma) from dark matter for years. Unfortunately, nobody else can detect this signal as Lauren Hsu from Fermilab explained in her review of dark matter experiments. One possibility is that their detector, which is made of iodine, is sensitive to dark matter particles but other chemical elements used by the other experiments were not. Two new experiments were built using iodine, COUPP and KIMS, and should soon have enough data to get the final word on this long-standing anomaly.

Dark matter might interact with the Higgs boson. If that’s the case, now that we have a mass for it, we can test specific hypotheses. The XENON100 experiment is just at the limit of sensitivity for this and new results will come soon.

This is a huge, open question in particle physics. Let’s hope the new (Higgs?) boson discovery will soon be followed by some clues on the nature of dark matter. Exciting times ahead.

Pauline Gagnon

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